Electric circuit for a capacitive sensor element

ABSTRACT

An electric circuit for capacitive sensor elements of a touch switch can be designed as a switch capacitor circuit and van be provided with a micro controller. The sensor elements are provided in sensor branches with a charging capacitor. In addition, a reference branch is provide. It contains a reference capacity and a reference charging capacitor, which have capacities that correspond to those of the sensor branches. By comparing the number of charge transfer processes required to reach the switching threshold in the sensor branch and the reference branch, fluctuations of the supply voltage, and consequently the switching threshold itself, can be compensated for.

FIELD OF APPLICATION AND PRIOR ART

[0001] The present invention relates to an electric circuit for at least one capacitive sensor element. Advantageously, the circuit controls such a sensor element in a touch switch and analyzes and recognizes triggering of the switch.

[0002] Such a circuit is known, for example, from U.S. Pat. No. 5,973,417. In that circuit, the complexity of the circuit has already been relatively reduced. The aim, however, is to further reduce the circuit complexity.

PROBLEM AND SOLUTION

[0003] The invention is based on the problem of providing a circuit as mentioned above and further reducing the circuit complexity for controlling and analyzing a capacitive sensor element. Furthermore, the susceptibility to interference is to be improved, in particular in case of fluctuations of the supply voltage for the circuit.

[0004] This problem is resolved by a circuit with the features of claim 1. Advantageous and preferred embodiments of the invention are the object of the other claims and will be described in more detail in the following. The wording of the claims is made with explicit reference to the content of the description.

[0005] According to the invention, the circuit is designed as the commonly known switch capacitor circuit for controlling and analyzing the sensor element. For this purpose, it is equipped with a micro controller. The sensor element is located in a sensor branch as part of the circuit. The sensor branch also contains the charging capacitor. According to the principle of the switch capacitor circuit, the sensor element is first charged in a first step, and the charging capacitor is in turn charged with the charge of the sensor element in a second step. The capacity of the sensor element is therefore chosen much lower than the capacity of the charging capacitor. For this reason, a large number of charge transfer processes are required until the charging capacitor is charged to a level that a certain potential is reached at the charging capacitor. This set potential is preferably the switching threshold of the micro controller or of the circuit, but it may also be a different potential.

[0006] If the sensor element is now triggered, its capacity increases significantly. In this manner, it can accommodate a higher charge and the number of required charge transfer processes onto the charging capacitor until it reaches the switching threshold is significantly lowered. This reduced number of charge transfer processes is detected and recognized as triggering of the sensor element or the touch switch.

[0007] In order to improve the circuit according to the invention, the circuit or the micro controller includes a reference circuit with a reference capacity. This reference capacity is in a known ratio to the sensor element in the non-triggered state. In addition, a reference charging capacitor is provided in the reference circuit. This creates a so-called reference branch.

[0008] According to the invention, the reference capacity is charged in a first step, similar to the case of the sensor element. This charge is transferred to the reference charging capacitor in a second step until the switching threshold of the reference charging capacitor is reached. The ratio between reference capacity and sensor element is known. It is then possible to compare the number of charge transfer processes in the reference branch, under consideration of the ratio between sensor element and reference capacity, with the number of the charge transfer processes in the sensor branch. This can eliminate the fluctuation in the supply voltage, and thus the switching threshold. According to the invention, greater fault protection of the circuit is achieved with a very simple design. The greater independence from fluctuations in the supply voltage, in particular, has the advantage that the power supply of the circuit can be designed more simply. This can lead to considerable cost savings. As can be seen from this, the main advantage of such a reference circuit is that interference with the circuit through one source or another can be suppressed or compensated for. For example, temperature drifting can be compensated for because the charging capacitor and the reference charging capacitor drift in the same direction.

[0009] Normally, fluctuation of the supply voltage has the disadvantage that it affect the switching threshold as well. If the switching threshold changes, the number of necessary charge transfer processes changes as well. This in turn makes clear detection of the triggering difficult. The fluctuation of the supply voltage for simple circuits can be up to 20%. This already significantly influences the switching threshold, and therefore the functional precision of a touch switch, for example.

[0010] Instead of the switching threshold, it is possible to use any arbitrary potential as the comparison value, as described above. For example, a comparator or an AD converter can be employed to use a different value. The switching threshold can be a certain potential, with respect to 0 Volt. Alternatively, it can be reached by a potential decreasing from 5 Volt, for example.

[0011] The reference capacity can be provided in the micro controller itself, for example determined by the construction. In this case, it is sufficient to determine the reference capacity for a certain type of micro controller or to determine the previously-mentioned ratio to the capacity of the sensor element.

[0012] Alternatively, a separate reference capacitor can be provided as a reference capacity. In this case, it suggests itself to implement the reference circuit in a separate reference branch. It is advantageous to design this branch basically identically to a sensor branch. For this purpose, the reference capacity can be provided as a capacitor connected to zero. In addition, there is a reference charging capacitor with a capacity that preferable corresponds to the capacity of the charging capacitor of the sensor branch.

[0013] Preferably, the reference capacity, in particular when using a separate reference capacitor, is chosen the order of magnitude of the non-triggered sensor element. Most preferably, they are equal. The result is a basically equal number of charge transfer processes in the non-triggered state of the sensor element until the switching threshold is reached.

[0014] Such a capacity of the sensor element or the reference capacitor can be in the range of several pF in the non-triggered state. Approx. 5 pF is considered advantageous. The capacity of the charging capacitor is chosen to be much higher again. It is between 10 nF and 1 μF, for example. Approx. 100 nF is considered particularly advantageous. The above-mentioned supply voltage of the micro controller is advantageously about 5 Volt.

[0015] In further embodiments of the invention, a micro controller can contain several sensor branches. It is possible, for example, to provide several sensor elements for one micro controller, for example one closed control unit and a number of touch switches.

[0016] In addition, the circuit can contain at least one resistor to improve the EMC properties. In this manner, interferences may be dampened. Advantageously, the resistor is provided in the reference circuit. It is also possible, however, to provide a resistor in the sensor branch.

[0017] The sensor element could basically be any capacitive sensor element. Advantageous is a three-dimensional, spatially deformable body made of conducting synthetic material, such as foam plastic. Such a sensor element for example is described in U.S. Pat. No. 5,917,165, the respective content of which is incorporated into this description by explicit reference. A sensor element is most preferably designed to be pressed on the bottom of a cover. The top part of the cover in this case can be a touch surface for the touch switch.

[0018] In one possible embodiment of the invention, several sensor branches can be connected each to its own input or port of the micro controller. This embodiment is advantageous if the number of sensor branches is not too high, for example two to four.

[0019] In an alternative embodiment of the invention, in particular when using a large number of sensor branches, several sensor branches can be connected to the micro controller via switches in a multiplex design. This design permits a very large number of sensor branches or sensor elements per micro controller. The switches used can be of different types. Advantageously, they are analog switches. Transistors or relays may be used as well. Providing such switches and multiplexing is not very costly or complex. Preferably, the reference circuit is also connected to the micro controller with a switch. In this case multiplexing is not required. This can compensate for the effects of the switches, which generally have a certain capacity, for example.

[0020] This and other properties can be seen not only in the claims but also in the description and the drawings, wherein individual characteristics may be used either alone or in sub-combinations as an embodiment of the invention and in other areas and may individually represent advantageous and patentable embodiments, for which a patent claim is made here. Embodiments of the invention are shown in the drawings and explained in detail in the following.

SHORT DESCRIPTION OF THE DRAWINGS

[0021] Embodiments of the invention are shown in the drawings and explained in detail in the following. The drawings show:

[0022]FIG. 1 a representation of a circuit with sensor branch and reference branch, during the discharge of the capacitors,

[0023]FIG. 2 the circuit in FIG. 1 during charging of the sensor element,

[0024]FIG. 3 the circuit in FIG. 2 when transferring the charge from the sensor element to the charging capacitor, and

[0025]FIG. 4 a sample circuit with a large number of sensor branches, which are connected to the micro controller via analog switches and multiplexing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] The sample circuits in the figures are schematic and have been reduced to the essential functional elements required to understand and explain the invention.

[0027] In FIGS. 1 to 3, one circuit 11 is represented in each case, which is designed according to an embodiment of the invention. In this case, a micro controller 13 is provided, which is connected to a supply voltage VS. A reference branch 15 is connected to two ports of the micro controller 13 on the left. This branch consists of a reference charging capacitor C_(RL) and a reference capacitor C_(R) connected to zero.

[0028] On the right side of the micro controller 13, a sensor branch 17 is connected to two additional ports. It is also possible to provide additional identical sensor branches here.

[0029] The sensor branch 17 has a capacitor C_(L) as charging capacitor, as in the reference branch 15. In addition, it contains the sensor element C_(S). It is shown partially dashed. This should illustrate that basically only in the triggered state, i.e. when the finger of a user makes contact for example, the sensor element C_(S) is a capacitor connected to zero.

[0030] The supply voltage can be approximately 5 Volt. The capacity of the charging capacitors C_(L) and C_(RL) can be approximately 100 nF. The capacity of the sensor elements C_(S) and the reference capacitor C_(R) can be approximately 5 pF, for example.

[0031] With the dotted line in the micro controller 13, different internal switch states can be represented. FIG. 1 shows how all the capacitors are discharged. FIG. 2 shows how the sensor elements C_(S) and the reference capacity C_(R) are each charged to the supply voltage VS.

[0032] In the next step in FIG. 3, the charge is transferred from the sensor elements C_(S) and the reference capacity C_(R) to the charging capacitors C_(L) and C_(RL), respectively. As the capacity of the charging capacitor is many times higher, in case of the selected values by a factor 20,000, several thousand charge processes are required to reach the switching threshold of the micro controller 13. This threshold is generally approximately 1.3 Volt. As soon as the charging capacitors are charged to the said 1.3 Volt after a sufficient number of charge transfer processes, the micro switches. The number of charge transfer processes can be determined in advance.

[0033] If the sensor element C_(S) is now triggered, its capacity increases significantly. This means that its capacity is much higher than the reference capacity C_(R). Consequently, it can store bigger charge during the charging processes, which is then transferred to the charging capacitors C_(L). When the sensor element C_(S) is triggered, more charge can be transferred to the charging capacitors with fewer charge transfer processes. As a result, it reaches the switching threshold with fewer charge transfer processes than the reference branch. As it is no longer necessary to determine the absolute number of charge transfer, processes, but rather the number of charge transfer processes in the sensor branches is compared with that of the reference branch, effects of the supply voltage fluctuation on the switching threshold can be disregarded. Furthermore, it can be seen that the circuit 11 basically works according to the principle of the switch capacitor circuit.

[0034] The micro controller 13 can be connected to a large number of sensor branches 17 as long as free ports are available.

[0035] With the shown circuit according to the invention, the switching threshold is no longer required as an absolute value. By comparison with the reference branch, the values of which are in a certain ratio with the sensor branch, or equal to it in a triggered state, the switching threshold of the micro controller is a guide value for both circuit branches and its absolute value is therefore not important.

[0036] In further embodiments of the invention it is also possible not to have a separate element for the reference capacitor C_(R). An existing capacitance in the micro controller 13, which cannot be completely avoided, can be used as well. Even if it has a capacity that is different from the sensor element C_(S), it is at least possible to determine the ratio between the two. This ratio can be taken into account during the comparison of the number of required charge transfer processes until the switching threshold is reached.

[0037] Another option of controlling several sensor branches with one micro controller 13 if the controller does not have a large number of ports is shown in FIG. 4. In this case, several sensor branches 117 are provided. They are each connected to a port of the micro controller 113 via analog switches 119. The analog switches 119 are controlled by a controller 120. This control is preferably done according to a multiplex method, which does not need to be described in detail here because it can be implemented according to a well-known design. With such a circuit 111 according to FIG. 4, a large number of sensor elements Cs can be operated by one simple micro controller 113.

[0038] In addition to the sensor elements C_(S), resistors 122 and capacitors 123 are provided in the sensor branches in order to improve the interference properties, more particularly the EMC properties. As known from existing designs, a charging capacitor C_(L) is provided.

[0039] In order for the reference branch 115 to behave basically in the same manner as the sensor branches 117, an analog switch 119 is provided there in addition to the reference capacitor C_(R) and the reference charging capacitor C_(RL). However, this switch may remain closed permanently because it is not affected by the multiplexing.

[0040] Another advantage of the provided reference branches 15 and 115 is that it is possible to compensate for the temperature sensitivity of the charging capacitors C_(L).

[0041] Advantageously, the measurement is performed during the zero crossing of the supply voltage. This has the advantage, in particularly for power supplies that are not galvanically separated, that the potential difference between the sensor element and zero, which is the installation frame of a cooking element for example, is very low or approaches zero. 

1. An electric circuit for at least one capacitive sensor element, wherein: the circuit is designed as a switch capacitor circuit for the sensor element and is provided with a micro controller, the circuit is provided with at least one sensor branch, the sensor branch contains the sensor element and a charging capacitor, the sensor element has a non-triggered state and a charged state, the charging capacitor is charged with the charge of the sensor element until it reaches a certain voltage, the sensor branch is connected with the micro controller, the micro controller is provided with a reference circuit the reference circuit has a reference charging capacitor and a reference capacity which are in a known ratio with the capacity of the sensor element in the non-triggered state, and the micro controller compares the number of the charge processes in the sensor branch with the number of the charge processes in the reference circuit until a certain voltage is reached.
 2. A circuit according to claim 1, wherein the reference capacity is provided in the micro controller.
 3. A circuit according to claim 1, wherein the reference capacity is a separate reference capacitor and the reference circuit is a separate reference branch.
 4. A circuit according to claim 3, wherein the reference branch is identical to the sensor branch.
 5. A circuit according to claim 1, wherein the reference capacity corresponds approximately to the capacity of the sensor element.
 6. A circuit according to claim 1, wherein the capacity of the sensor element in the non-triggered state is in the range of several pF.
 7. A circuit according to claim 1, wherein the capacity of the charging capacitor is between 10 nF and 1 μF.
 8. A circuit according to claim 1, wherein the capacity of the reference charging capacitor is between 10 nF and 1 μF.
 9. A circuit according to claim 1, wherein the micro controller has a plurality of sensor branches.
 10. A circuit according to claim 1, wherein there is provided a resistor in the circuit to improve the EMC properties.
 11. A circuit according to claim 10, wherein the resistor to improve the EMC properties is provided in the reference circuit.
 12. A circuit according to claim 1, wherein the sensor element is a three-dimensional, spatially deformable body made of conducting synthetic material, wherein the sensor element is designed to be in contact with a bottom of a cover, wherein the top of the cover is the touch surface for a touch switch.
 13. A circuit according to claim 1, wherein the micro controller is provided with inputs and several sensor branches are connected each to its own input of the micro controller.
 14. A circuit according to claim 1, wherein several sensor branches are connected to the micro controller with multiplexing via one analog switch each.
 15. A circuit according to claim 14, wherein the reference circuit is connected to the micro controller via an analog switch. 